I had a link to email me and the spam harvesters got ahold of it. So now there's no way to send corrections, additions, etc... Sorry.

What is a geophone?A geophone is
a small, cheap instrument for measuring ground motion. There are many
different varieties for different applications. They are designed for
earthquakes, machine vibrations, oil exploration, mining, etc...

How much are they?It depends on a lot of different factors. Basically the price is between free to
beyond $1000.00 U.S.. I have two used 10 Hz geophone kits
that I bought for about $10.00 U.S. each. The more
complicated, lower resonant frequency, multiple channel, name brand,
new geophones cost more money. The older, simpler, single channel,
used geophones are cheaper. I don't sell geophones.

What does it have to do with ghost hunting?
Ghosthunters is now using a geophone or possibly an accelorometer to
detect vibrations so they can be recorded by the camera. Their
geophone is only acting as a detector. They don't digitize the
signals to be analyzed or interpreted, except with the camera.
Normally the amplified signal from a geophone would be digitized and
recorded or just recorded to paper with a chart recorder. The kit
they use on Ghosthunter looks like something you might find at BG
Micro, eBay or maybe United Nuclear. I would guess the value at around $30 USD, if you're shopping for something similar.

What's the difference between all of the different models?There are huge differences between the models and options for geophones. The exterior case is optional on a
lot of geophones. Some have coaxial connectors and some have binding
post connectors, but most have two little pins that you connect your
leads to. The resonant frequency is one of the main factors in the
price. Lower resonant frequencies are more difficult to achieve in a
small box with a light weight and a low price. Basically you want the
resonant frequency to be close to what your looking for in signals.
Also your application should be a factor. You probably don't need a 1
Hz resonant frequency to watch local earthquakes, but you would want
one for distant earthquakes. The frequency response of an instrument
is probably centered around the resonant frequency and is very narrow
in width.

What do they look like?They look like little metal cylinders mostly. They vary in size and proportions
with different connectors and some go into little cases. There are
some pictures on the web sites listed elsewhere on this page.

How do they work?Most of the models have a coil hanging from a spring in the center of some magnets. When
the case is moved up and down the mass tends to stay put and induces
small currents into the coil as it moves through the magnetic field.
It measures velocity of motion. The current is used to move a pen
over paper or gets recorded into a computer for analysis.

Is a geophone the same thing as an
accelerometer or seismometer?Not really. Geophones would sit
in between accelerometers and seismometers in function and price.

Seismometers are typically larger and more expensive. They usually
detect extremely small movements at lower frequencies than geophones.
Seismometers can be very fragile and sensitive, although they
are designed to survive earthquakes, that may be from hundreds of
miles away. Calibrating a seismometer might be critical to getting
useful data out of it and a geophone or accelerometer would more
likely to be used just to get a simpler signal.

Accelerometers are nearly solid state and good at handling more
violent motion. Video games and cell phones are now using
accelerometers to interact with the users as another input device.

All three might have a intended axis or orientation (XYZ or
Mercator plus vertical) or they might be a bundle of more than one
axis in the same package. Single axis instruments are limited in
function off of intended axis.

What are spurious frequencies?This
information was given to me and I tried to paraphrase it for
simplicity. Basically, it's the noise from high frequency
oscillations in the suspension spring. The spring inside the geophone
that supports the mass, can pick up higher frequencies and add noise
to the signal. Some values I saw while poking around are pretty high
and would not effect seismic signals. At least not distant seismic
events.

How old are geophones? When was the first one
made?Real seismometers go back to the 1900's. Geophones
probably came shortly after.

Mine says "Pollard" on it or looks
like a stethoscope on one end.It's not a typical geophone
with a coil and a magnet. It's for listening for underground leaks in
pipes. Here's a good link.

What types of information can you get out of a geophone?

frequency spectra of an event

presense/absense

event time and duration with a reference clock

phase

amplitude (relative or calibrated)

orientation and direction by using multiple instruments with triangulation or phase (XYZ)

distance using accurate timing and larger distances between the geophones (triangulation)

distance using signal amplitude of instruments that are widely dispersed (proximity)